Insulin receptors and responsiveness have been studied in undifferentiated and fatty 3T3-L1 fibroblasts. With differentiation, 3T3-L1 fibroblasts develop lipoprotein lipase. Insulin and insulin receptor antibody stimulate release of lipase from the cells as well as increases intracellular lipoprotein lipase activity. Antibody to the insulin receptor mimics insulin in the 3T3-L1 cells in that it stimulates glucose transport and oxidation as well as rendering these cells insulin resistant. Recovery of the cells from antibody-induced insulin resistance has also been studied.